The possibility of reconstructing the velocity structure of inspected objects with a high spatial resolution and high sensitivity in ultrasonic tomographic nondestructive testing within the framework of a wave model has been demonstrated in a real experiment. In this study, a scheme of a tomographic experiment with rotation is proposed, which provides sounding of an inspected object from multiple sides. A tomographic scheme of the experiment with linear antenna transducer arrays operating at a frequency of ∼5 MHz was used. The experiment was conducted on dedicated samples including inserts with different sound propagation velocities. It was shown that the velocity structure and the boundaries of inserts can be reconstructed in the transmission and the reflection schemes. The reconstruction of the velocity structure was formulated as a nonlinear coefficient inverse problem for a scalar wave equation. Efficient iterative methods for its solution on a supercomputer were developed using direct formulas to compute the gradient of the residual functional between the computed and experimentally measured wave field at the detectors. Nonlinearity of the inverse problem of ultrasound tomography leads to multiple local minima of the residual functional. A two-stage iterative method was used for velocity reconstruction. The transmission scheme enables a spatial resolution of approximately 1 mm with a velocity contrast of 2%.
Keywords: Material characterization; Nonlinear inverse problems; Tomographic imaging; Transducer antenna array; Ultrasonic tomography; Wave equation.
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